The present invention is directed to a manufacturing method for printed wiring boards having electromagnetic wave shielding.
In order to prevent the malfunction of a circuit which may be caused by electromagnetic wave between the associated printed wiring circuits or between one printed wiring board and an external device, a printed wiring board is widely known having an electromagnetic shielding provided over the entire surface or local portion thereof through insulating layers.
Electrically conductive paste is usually applied on the printed wiring circuit through a silk screen by means of a printing method, and then cured to obtain the electromagnetic wave shielding layer.
Also, the electromagnetic wave shielding layer is electrically connected with the printed wiring circuit by way of a ground circuit of the printed wiring circuit, thereby accomplishing the electromagnetic wave shielding.
Referring now to FIG. 2 for further concrete illustration of the structure of the printed wiring boards described above, a desired printed wiring circuit 2 is formed on an electrically insulating substrate 1 by the conventional known method (for example, by etching copper foil of a copper-clad laminate), and then the electromagnetic wave shielding layer 6 is formed above the printed wiring circuit 2 through an insulating layer 3.
The electromagnetic wave layer 6 is electrically connected with a ground circuit 4 of the printed wiring circuit 2 through openings 4a defined by the insulating layer 3.
As particularly shown in FIG. 2, for the case where a multiplicity of connection areas 5 are densely formed between the electromagnetic wave shielding layer 6 and the printed wiring circuit 2, and for the case where the electromagnetic shielding layer 6 is formed by applying an electrically conductive paste through the silk screen, a conventional method uses an insulating layer composed of a plurality of layers so as to define the openings 4a whose section is of a cup shape, thereby ensuring an easy filling of the electrically conductive paste into the openings 4a.
As is clearly shown in FIG. 2, the insulating layer 3 includes a first, a second and a third insulating layers 3a, 3b and 3c which define stepwisely the opening 4a having toward the top a greater diameter, thereby imparting the configuration of a cup to the opening 4a.
In consequence, when the electrically conductive paste for the electromagnetic wave shielding layer 6 is applied through thus formed opening 4a, the electrically conductive paste can be smoothly filled into each of the openings 4a having a cup shape, thereby ensuring a secure connection between the electromagnetic wave shielding layer 6 and the printed wiring circuit 2 through each of the ground circuits 4.
However, in the conventional printed wiring board 11, at least three insulating layers are formed on the ground circuit 4 when densely forming a multiplicity of connection areas 5. This brings about a considerable unevenness between the insulating layer 3 and the ground circuit 4, and thickens the electrically conductive paste present in the connection area 5 of the ground circuit 4 at the time of forming the electromagnetic shielding layer 6.
The electrically conductive paste having the increased thickness is cured using the process of ED curing (electron radiation curing) which may produce an uncured portion depending on the curing time and irradiation amount. Moreover, the thickness of the electromagnetic wave shielding layer 6 is reduced due to the enlarged unevenness of the electromagnetic wave shielding layer 6 resting on the insulating layer 3, which may lead to problems decreasing the effectiveness of the electrical connection, safety of conductive resistance and the like.
When the printed wiring boards 11 delivered to users are subjected to heat during the soldering for mounting electrical components thereon, if the electrically conductive paste is made thicker, a thermal expansion during the soldering will cause distortion therebetween due to expansion and contraction of the individual materials, which may impair the adhesion properties of the electrically conductive paste.
Furthermore, with the greater film thickness, the components mounted on the connection area 5 extending along the end surface of the printed wiring board 11 are subjected to cracking due to the impact at the time of contour processing using a press, which may also lead to peeling.